Loop not breaking (Javascript)?

Question

I'm trying to write a simple program with JavaScript that takes in a number 1 - 100 and displays it with it's letter grade as compared to the American standard of grading. (<50 = F, 60 - 69 = D, 70 = 79 = C, etc...). I am having an issue where the script doesn't register the "STOP" command I put into it and instead, takes it as an invalid input. I want to use "STOP" to break out of the loop and any other NaN entry to throw an Error. Here is my code:

        var entry;
    var letterGrade;

    while(true) {
        entry = prompt("Enter number grade from 0 through 100\n" +
                       "Or enter STOP to end entries", "STOP");
        entry = parseInt(entry);

        // derive and display the letter grade
        if(isNaN(entry)){
            if(entry === "STOP"){
                letterGrade = 'Q';
            }
            else{
                letterGrade = 'N';
            }
        }

        else if(entry < 0 || entry > 100){
            letterGrade = "ERROR";
        }
        else if(entry < 60){
            letterGrade = 'F';
        }
        else if(entry > 59 && entry < 70){
            letterGrade = 'D';
        }
        else if(entry > 69 && entry < 80){
            letterGrade = 'C';
        }
        else if(entry > 79 && entry < 90){
            letterGrade = 'B';
        }
        else{
            letterGrade = 'A';
        }

        if(letterGrade == 'Q'){
            break;
        }
        else if(letterGrade == 'N'){
            alert("ERROR #002: Entry is not a number");
        }
        else if(letterGrade != "ERROR"){
            alert("Number grade = " + entry + "\n"
                + "Letter grade = " + letterGrade);
        }
        else{
            alert("ERROR #001: Invalid Entry");
        }
    }

Answer 1

It would be far better practice to use a recursive function here:

function enterGrade() {
    // All your other code above and below next line as it is above
    if(!isNan(entry)) enterGrade();
}

I can add a bit more code if you don't get what I'm trying to explain.

Answer 2

entry = prompt("Enter number grade from 0 through 100\n" +
                       "Or enter STOP to end entries", "STOP");
        entry = parseInt(entry);

You can't have it both a string value and an integer. Since you reassign entry as an int (or attempt to):

if(isNaN(entry)){
            if(entry === "STOP"){

Will never match. You need to rearrange things as such:

 var entry;
var letterGrade;

while(true) {
    entry = prompt("Enter number grade from 0 through 100\n" +
                   "Or enter STOP to end entries", "STOP");

    // derive and display the letter grade
    if(isNaN(entry)){
        if(entry === "STOP"){
            letterGrade = 'Q';
        }
        else{
            letterGrade = 'N';
        }
    }
    entry = parseInt(entry); // parse to int here
    if(entry < 0 || entry > 100){
        letterGrade = "ERROR";
    }
    else if(entry < 60){
        letterGrade = 'F';
    }
    else if(entry > 59 && entry < 70){
        letterGrade = 'D';
    }
    else if(entry > 69 && entry < 80){
        letterGrade = 'C';
    }
    else if(entry > 79 && entry < 90){
        letterGrade = 'B';
    }
    else{
        letterGrade = 'A';
    }
    if(letterGrade == 'Q')
       break;
}